Why can a WideString not be used as a function return value for interop?

Question

I have, on more than one occasion, advised people to use a return value of type WideString for interop purposes.

• Accessing Delphi DLL throwing oc

Answer 1

In C#/C++ you will need to define the Result as out Parameter, in order to maintain binary code compatibility of stdcall calling conventions:

Returning Strings and Interface References From DLL Functions

In the stdcall calling convention, the function’s result is passed via the CPU’s EAX register. However, Visual C++ and Delphi generate different binary code for these routines.

Delphi code stays the same:

function TestWideString: WideString; stdcall;
begin
  Result := 'TestWideString';
end;

C# code:

// declaration
[DllImport(@"Test.dll")]        
static extern void  TestWideString([MarshalAs(UnmanagedType.BStr)] out string Result);
...
string s;
TestWideString(out s); 
MessageBox.Show(s);